Computational analysis of a confined capillary interface under the influence of a perturbative rotating impulse

The following thesis discusses two separate microfluidic applications analysed with ANSYS Fluent 2022R1. For a rotating capillary tube of a diameter of 2 mm and height of 10 mm that is filled with air on the top half and water on the bottom half $(Bo_g=\frac{\Delta\varrho g L^2}{\sigma}\approx0.543)$ applying different rotational velocity profiles to the cylinder wall was found to not have a significant impact on the fluid interface. The geometry was replicated with a 2D axisymmetric model and analysed with and without the effect of gravity, trying different stabilization methods and settings. For constant time stepping the instability region was identified, for adaptive time stepping the results showed an almost complete independence from the imposed wall speed. To instable setups the addition of stabilization mechanisms strong enough to overcome known issues within the software led to unphysical results. A liquid combination with densities and viscosities lying closer to one another showed a similar behaviour. It was not possible to replicate the results leading to this research showing capillary breakup through different mode shapes and thereby an aerosol formation. The methods to possibly replicate the simulations, also in a three-dimensional approach, were elaborated. For a glass substrate of a diameter typical for the semiconductor industry of 40 mm, the apparent contact angle between the liquid droplet and solid was estimated from literature and simulations performed for both cases of static and dynamic spin coating. By rotating around 0.1 s all three analysed liquids being Oil, Water, and Ethanol were entirely spun off the surface without leaving a liquid film behind. A method to quantify the maximum radial extension of the fluid as well as the angle of the liquid-air interface through Fluent macros and MATLAB was presented. It was found that for the same angular speed $\omega$ and rotational Bond number $Bo_r$ Water droplets are the most prone to fingering instabilities, followed by Ethanol and then Oil. This phenomenon was characterized through three dimensionless numbers describing the problem, being $Bo_r$, $We$ and $Oh$. While the island detachment in fingering instability seems to be independent of $Oh$ in the static case, there was found to be optimization potential in terms of combinations of $Bo_r$ and $We$ in the dynamic one. Higher angular speeds generally lead to quicker detachments, while the right droplet impact velocities could be able to counter this effect. The initial phase of the liquid spreading was found to depend solely on We, hence being uncoupled from the substrate’s angular speed. The specific interval of ${10}^1<Bo_r<{10}^3$ was elaborated for static spin coating with Ethanol droplets of different diameters at varying angular speeds. The obtained results within this interval present a good fit with dimensionless results from previous experimental data. They also point towards a horizontal shift of the curves relating the dimensionless diameter to $Bo_r$ for the same liquid at different rotational speeds.